Microcapsules can be constructed of various types of wall or shell materials to house varying core material for many purposes. The encapsulation process is commonly referred to as microencapsulation. Microencapsulation is the process of surrounding or enveloping one substance, often referred to as the core material, within another substance, often referred to as the wall, shell, or capsule, on a very small scale. The scale for microcapsules may be particles with diameters in the range between 1 and 1000 μm that consist of a core material and a covering shell. The microcapsules may be spherically shaped, with a continuous wall surrounding the core, while others may be asymmetrical and variably shaped.
General encapsulation processes include emulsion polymerization, bulk polymerization, solution polymerization, and/or suspension polymerization and typically include a catalyst. Emulsion polymerization occurs in a water/oil or oil/water mixed phase. Bulk polymerization is carried out in the absence of solvent. Solution polymerization is carried out in a solvent in which both the monomer and subsequent polymer are soluble. Suspension polymerization is carried out in the presence of a solvent (usually water) in which the monomer is insoluble and in which it is suspended by agitation. To prevent the droplets of monomers from coalescing and to prevent the polymer from coagulating, protective colloids are typically added.
Through a selection of the core and shell material, it is possible to obtain microcapsules with a variety of functions. This is why microcapsules can be defined as containers, which can release, protect and/or mask various kinds of active core materials. Microencapsulation is mainly used for the separation of the core material from the environment, but it can also be used for controlled release of core material in the environment.
Microencapsulation has attracted a large interest in the field of phase change materials (PCMs). A PCM is a substance with a high heat of fusion, melting and solidifying at a certain temperature, which is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent heat storage units. The latent heat storage can be achieved through solid-solid, solid-liquid, solid-gas and liquid-gas phase change, but solid-liquid is typically used in thermal storage applications as being more stable than gas phase changes as a result of the significant changes in volume occupied by the PCM. Because of this ability, PCMs are currently being used in a wide variety of fields including textiles, food and medical industries, computer cooling, spacecraft thermal systems, and solar power plants. Generally, the most commonly used PCMs in use today are those made from paraffin waxes. Additionally, because PCMs transition from solid to liquid when heated past the melting point, paraffin waxes are most easily handled when encapsulated, with the most common outer wall being an organic polymer. This allows PCMs to be handled as free-flowing solids past the melting temperature of the PCM, and the organic polymer wall improves controlled release of the PCM, if that is desired, and structural stability of the capsule.
Some disadvantages exist in current organic polymer wall systems of the microencapsulated PCMS, including flammability (too high), low far infrared (FIR) absorption, little to no defense against bacterial and fungal growth, and low thermal conductivity. Previously, to combat these limitations, researchers have tried direct encapsulation of PCMs with inorganic walls, such as calcium carbonate (CaCO3), silica, aluminum hydroxide (Al(OH)3), and oxides of metals such as Mg, Ca, Ti, and Zn, but the walls have been ineffective at containing the PCM. In particular, a major issue with this type of direct encapsulation is the amount of PCM that leaks from the capsule, as much as 30% leakage. Leakage of the PCM in such quantities, especially when the PCM is a paraffin wax, could increase the flammability of the microcapsules. Furthermore, in order to obtain a complete wall of inorganic material encapsulating the paraffin core, a mass ratio of around 40/60 (wax core/wall) must be used. This high mass ratio causes a nearly 60% loss in enthalpy, which significantly lowers the ability to effectively use the PCM core for many of the applications mentioned above. Therefore, wall materials are limited to organic polymers.
Some further potential applications of PCMs include heating/cooling systems in buildings as well as solar energy storage. Efficient heating and cooling systems in buildings have come a long way in recent years; however, there is still room for improvement. Because of PCMs' ability to store and release heat when needed, PCMs have applications in heating/cooling systems in buildings. However, due to the flammability of organic PCMs, the applications are limited. Additionally, solar panels are becoming much more efficient at energy conversion; however, a method of storage of this energy for later use is needed. Energy is released in the form of FIR light from the sun, and radiates both during day and night. Because of this, a material that is able to absorb FIR energy and store it as heat would be desirable in solar energy applications. PCMs have the ability to store and release heat over longer periods of time.
Since the development of microencapsulated PCMs, there has been a constant need for improved microcapsules. In particular, there is a need to find a way to use inorganic materials as walls of microcapsules in a way to get the benefits of the inorganic material without leakage of the core and without decreasing the heat of fusion of the microcapsule.